1. Field of the Invention
The present invention relates to a power-assisted steering system for automotive vehicles, and more particularly to a power-assisted steering system of the type which includes a hydraulic reaction mechanism for applying a reaction force to the driver's steering effort in accordance with travel speed of the vehicle.
2. Description of the Prior Art
Disclosed in Japanese Patent Publication No. 49-30659 is a power-assisted steering system of this kind wherein hydraulic fluid under pressure from a hydraulic fluid pump is supplied through a fixed orifice into a hydraulic reaction mechanism in a changeover valve unit and is controlled in pressure in accordance with travel speed of the vehicle. FIG. 5 illustrates a relationship between the driver's steering effort and the pressure of hydraulic fluid supplied into a power cylinder under control of the changeover valve unit. In the right-hand part of FIG. 5, there are illustrated a relationship between various steering angles in relation to travel speed of the vehicle. For applying a natural and appropriate reaction force to the driver's steering effort during travel of the vehicle at low and high speed, it is necessary to provide a linear relationship between the driver's steering effort and the power cylinder pressure and to moderate a rise gradient of the power cylinder pressure relative to the driver's steering effort in accordance with an increase of the vehicle speed. In a region where turning maneuvers of the vehicle are effected at a large steering angle under a high gravity or in an emergency, it is necessary to cause a quick increase of the power cylinder pressure to reduce the driver's steering effort.
In the case that appropriate control of the power cylinder pressure may not be effected in accordance with the type of vehicles a sudden change in the driver's steering effort will occur in turning maneuvers of the vehicle. This causes an unstable feel in steering operation and results in excessive turning of the vehicle. In the steering system, however, it is unable to restrain a sudden increase of the power cylinder pressure in turning maneuvers of the vehicle, and it is also unable to control the rise gradient of the power cylinder pressure in accordance with travel speed of the vehicle.